Land application of treated wastewater sludge, or biosolids, is controversial because the material therein contains human pathogens. Wastewater treatment agency managers are concerned about the future of biosolids land applications in their states. The United States Environmental Protection Agency's (EPA's) Class A pathogen standards require the virtual elimination of pathogens in biosolids. In accordance with 40 CFR § 503.32 (a)(5), Class A—Alternative 3 and 40 CFR § 503.32(a)(6)—Alternative 4, EPA Class A pathogen requirements are met in biosolids when fecal coliform densities are less than 1,000 most probable number (MPN) per gram total solids; or when Salmonella densities are less than 3 MPN per four grams total solids. Enteric virus density must be less than one plaque-forming unit per four grams of total solids, and helminth ova is less than one viable helminth ova per four grams of total solids. Additionally, EPA provides time and temperature requirements under 40 CFR § 503.32(a)(3—Alternative 1, that state the required reduction in pathogen densities.
Meeting Class A standards will significantly increase the opportunity for biosolids recycling, however, known processes which achieve Class A pathogen densities in biosolids are generally costly, and in some instances, cost prohibitive. The provision of a low-cost method of biosolids treatment, which will meet Class A standards will offer an additional biosolids management option. Known thermophilic anaerobic digestion processes are not classified as a Process to Further Reduce Pathogens (PFRP) under 40 CFR Part 503.
In Class A sludge land application, three main criteria need to be met:
1) Pathogen Kill. This is accomplished by the BFT3 process of the invention;
2) Vector Attraction Reduction (VAR). This refers to the degree that the finished Biosolids (treated sludge) have been stabilized. Unstable solids will attract flies and other vectors as well as creating nuisance odors. This criterion must be met equally by both Class A and Class B materials. A major regulated criterion is that at least 38% of the volatile solids must be destroyed in anaerobic digestion. This can be reliably accomplished by holding sludge in an anaerobic digester for certain mean detention times, depending on the process used. For example, most mesophilic-only anaerobic systems operate in the 35° C. range and typically are designed for 20-day mean cell residence time (MCRT), and could possibly still meet the criteria in as little as 12 to 15 days. A thermophilic-only system might be designed for a 15-day MCRT and could possibly meet the criterion as little as 7 to 10 days. Temperature-phased systems, e.g., thermophilic treatment followed by mesophilic treatment, have been designed for as little as 12 days total MCRT.
3) Pollutants. This criterion refers to the concentrations of certain regulated pollutants and metals in the finished biosolids. This is usually a function of the industries that discharge to a wastewater system more so than any process used in treating the wastewater sludge.
Anaerobic digestion has been one of the most widely used processes for the stabilization of primary and secondary sludges produced at municipal wastewater treatment facilities. The majority of applications of anaerobic digestion to wastewater sludges have been in the mesophilic temperature range, from 35° C. to 40° C. (95° F. to 104° F.). Anaerobic sludge digestion in the thermophilic temperature range from 45° C. to 65° C. (113° F. to 149° F.) has been practiced to only a limited extent.
The limited use of anaerobic digestion at temperatures above the mesophilic range is due to higher energy requirements to obtain the higher thermophilic temperature, and because early reviews of such systems in the relevant literature identified problems, such as poor process stability, increased odor, and lower quality supernatant (filtrate/centrate). Many of these concerns have been proven to be untrue in currently-operating thermophilic systems. The advantages of thermophilic anaerobic digestion over mesophilic anaerobic digestion include increased stabilization and methane production rates, and improved sludge dewatering properties. Since the advent of the 40 CFR Part 503 Regulations, more studies have focused on the destruction of pathogenic organisms.
Thermophilic anaerobic digestion has an advantage of improving pathogen destruction, and has the potential to meet the pathogen quality requirements of EPA's Class A biosolids. The EPA Pathogenic Equivalency Committee (PEC) has stated that 2-log10 and 3-log10 reduction in pathogenic density of Ascaris ova and poliovirus, respectively, will be required in order to prove Class-A performance.
Research has shown that pathogen destruction in municipal sludge digestion follows a time/temperature relationship, wherein higher temperatures require shorter exposure times for pathogen destruction. Data have been collected demonstrating survival rates of various pathogens in municipal sludge digestion. These data suggest that thermophilic digestion achieves pathogenic bacteria reduction rates well in excess of two orders of magnitude higher than mesophilic digestion, and may meet the pathogen densities required for Class A sludge.
Anaerobic digestion has been performed in cylindrical or egg-shaped vessels for many years. However, the physical requirements for a pipeline, or plug-flow, digester, or reactor, are, as yet, unproven. While the 40 CFR, Part 503 regulation allows for use of plug-flow reactors to meet time and temperature requirements, the particulars of such a system are currently undefined. While standards are not currently defined for a thermophilic treatment process in a plug-flow reactor, one may be designed based on application of engineering and science.